This invention relates to an apparatus for the determination of unbalance in rotating bodies and in which the unbalance determination is carried out by an analysis with reference to phase and amplitude of the mechanical vibrations caused by rotating masses in unbalance. The mechanical vibrations are measured as motions, forces or pressures by means of transducers, which convert the mechanical vibrations to electrical signals. Each signal is the combination of fundamental oscillations caused by the unbalances, and noise.
A frequent problem that is encountered in apparatus of this type is that the noise signal may contribute a large part, sometimes even the most significant part of each electrical signal and, thereby, demanding an improvement of the signal to noise ratio before a proper analysis of the unbalances can be carried out.
A well-known method to improve the signal to noise ratio of the unbalance signal from a rotating body is to fix a known mass to the rotating mass, the unbalance of which must be determined. The electrical signal from the measuring transducer will then be the combination of signals caused by the unknown unbalance and the unbalance caused by the added mass, as well as the noise signal.
By moving the known mass to different points on the circumference of the rotating body and in each point measure the vibratory signal, the contribution to the signal by noise can be reduced and that by the known mass be eliminated by graphical and arithmetic means.
From the description of U.S. Pat. No. 3,147,624, it is known that a predetermined unbalance may be added electronically to an unknown unbalance signal and be moved in phase relatively to that signal in order to obtain the possibility to improve the signal to noise ratio without the application and change of additional mass to the rotating body.
Another well-known method for removal of noise signals is to use electronic filters which, however, may contribute large phase changes to the signal, if the speed of the rotating body deviates from the centre frequency of the filter.